A typical conventional transverse flux inductor comprises a pair of induction coils. A material to be inductively heated is placed between the pair of coils. For example, in FIG. 1, the coil pair comprises coil 101 and coil 103, respectively located above and below the material, which may be, for example, metal strip 90, which moves continuously through the pair of coils in the direction illustrated by the arrow. For orientation, a three dimension orthogonal space is defined by the X, Y and Z axes shown in FIG. 1. Accordingly the strip moves in the Z direction. The gap, gc, or opening, between the coil pair is exaggerated in the figure for clarity, but is fixed in length across the cross section of the strip. Terminals 101a and 101b of coil 101, and terminals 103a and 103b of coil 103, are connected to one or more suitable ac power sources (not shown in the figures) with instantaneous current polarities as indicated in the figure. Current flow through the coils creates a common magnetic flux, as illustrated by typical flux line 105 (illustrated by dashed line), that passes perpendicularly through the strip to induce eddy currents in the plane of the strip. Magnetic flux concentrators 117 (partially shown around coil 101 in the figure), for example, laminations or other high permeability, low reluctance materials, may be used to direct the magnetic field towards the strip. Selection of the ac current frequency (f, in Hertz) for efficient induced heating is given by the equation:
  f  =      2    ×          10      6        ⁢                  ρ        ⁢                                  ⁢                  g          c                                      τ          2                ⁢                  d          s                    
where ρ is the electrical resistivity (in Ω·m) of the workpiece; gc is the length of the gap (opening) between the coils in meters; τ is the pole pitch (step) of the coils in meters; and ds is the thickness of the strip (in meters).
The classical problem to be solved when heating strips by electric induction with a transverse flux inductor is to achieve a uniform cross sectional (along the X-axis), induced heating temperature across the strip. FIG. 2(a) illustrates a typical cross sectional strip heating profile obtained with the arrangement in FIG. 1 when the pole pitch of the coils is relatively small and, from the above equation, the frequency is correspondingly low. The X-axis in FIG. 2(a) represents the normalized cross sectional coordinate of the strip with the center of the strip being coordinate 0.0, and the opposing edges of the strip being coordinates +1.0 and −1.0. The Y-axis represents the normalized temperature achieved from induction heating of the strip with normalized temperature 1.0 representing the generally uniform heated temperature across middle region 111 of the strip. Nearer to the edges of the strip, in regions 113 (referred to as the shoulder regions), the cross sectional induced temperatures of the strip decrease from the normalized temperature value of 1.0, and then increase in edge regions 115 of the strip to above the normalized temperature value of 1.0. When the pole pitch of the coils is relatively large, from the above equation, the frequency is correspondingly high. In these cases under heating in the identified shoulder regions disappears while overheating of the edges remains as illustrated in FIG. 2(b). Typically a constant induced heating temperature across the entire cross section of the strip is desired so that, for example, under heated shoulder regions and overheated edge regions of the strip do not have to be scrapped when the heated strip undergoes a coating process.
Many solutions have been proposed to correct the edge heating problem, such as separate edge heaters, and arrangements of coils and/or laminations to alter the configuration of the resulting flux field, which in turn, attempts to alter the edge heating profile of the strip. While there may be some benefit to these approaches, there still exists the need for an arrangement of a transverse flux induction inductor that can achieve significant uniformity in cross sectional heating of the strip, particularly when the position of the strip varies in the coil or when the width of the strip varies.